Moisture measurement in a swelling clay soil using neutron moisture meters

Abstract

Neutron meters were calibrated for moisture determination in a layered swelling soil, showing changes in both swelling characteristics and texture with depth. The bulk density-moisture content relationship of each layer determined from small soil cores agreed with those predicted from in-situ swelling measurements taken during a previous study. The precision of calibration of two neutron meters was markedly improved by correcting for textural and bulk density changes with depth. The regressions of total moisture content on corrected neutron ratios were similar for the two moisture meters and comparable to those obtained for similar soils in previous studies, provided that the same calibration techniques were used. For moisture determination in the field, separate calibration equations of volumetric moisture content against neutron ratios were required to account for the individual bulk density-moisture content relationship of each soil layer. The calibration equation for the surface layer had a markedly lower calibration coefficient and a much higher intercept constant than for the subsoil. Measured changes in soil moisture following irrigation using the above calibration equations of the neutron moisture meter agreed closely with values predicted by water balance within a large soil core in a lysimeter.